Tropylium salts

If triphenylmethyl chloride in ether is treated with sodium, a yellow colour is produced due to the presence of the anionic spiecies PhsC". Alternatively, if triphenylmethyl chloride is treated with silver perchlorate in a solvent such as THF, the triphenylmethyl cation is obtained. More conveniently, triphenylmethyl salts, PhsC X", can be obtained as orange-red crystalline solids from the action of the appropriate strong acid on triphenylcarbinol in ethanoic or propanoic anhydride solution. The perchlorate, fluoroborate and hexafluoro-phosphate salts are most commonly used for hydride ion abstraction from organic compounds (e.g. cycloheptatriene gives tropylium salts). The salts are rather easily hydrolysed to triphenylcarbinol. 

Structure of tropones, tropolones, and tropylium salts with fused heterocyclic rings 95AHC(64)81. 

This method is a modification of the method originally published by Kursanov and Vol pin.6 Tropylium salts have also been prepared by bromination-dehydrobromination of tropili-dene,6 and by the hydride-exchange reaction between tropilidene and triphenylmethyl carbonium ion.7... 

Tropylium salts are starting materials for the preparation of a wide range of substituted tropilidenes. The fluoborate is the salt of choice for work involving the tropylium ion because it is indefinitely stable, non-hygroscopic, and, unlike the perchlorate, non-explosive. Its preparation by this method avoids the use of... 

That the photoreactive species is the carbonium ion and not the corresponding alcohol is clearly indicated by the relative concentrations of the two species present. The calculated equilibrium constant for 5% aqueous sulfuric acid implies an alcohol content of 2 7 x 10 %, much too low to account for any detectable photoreaction from this covalent species. In addition, when the tropylium salt is irradiated in the absence of acid, neither 3 nor 4 is detected as a product, but rather ditropyl (5) and its photoisomer (6) are observed. 

On the other hand, the anodic oxidation of 1,3,5-cycloheptatrienes is one of the most powerful key tools for the preparation of a variety of non-benzenoid aromatic compounds such as tropylium salts, tropones, tropolones, 2H -cyclohcpta h furan-2-oncs and azulenes14. 

The only studies on olefin polymerisations in methylene dichloride in which kp was deduced directly from the rate of reaction were carried out by Ledwith and his collaborators [9, 13] with extremely low concentrations of monomer and catalyst. They polymerised isobutyl vinyl ether and N-vinyl carbazole in a Biddulph-Plesch calorimeter with trityl or tropylium salts and obtained the first-order rate constants k1 from the conversion curves. Since different catalysts gave the same ratio of kx c they concluded that for each of them Xxr = c0 and hence identified with kp which must in fact be k p, as explained above. It seems unlikely that if several initiators give the same value of kp, they do so because they are all equally inefficient, and the inference that they do so because they are all 100% efficient, i.e., that for all of them x = c0, seems plausible - but it would be useful to have a direct check of this. 

Table 3 Propagation rate-constants for the polymerisation of NVC in CH2CI2 by tropylium salts ...

The allenylsilanes are excellent nucleophiles and they can react with a variety of electrophilic species in annulation processes that provide access to diverse products. Allenylsilane 112 (Eq. 13.36) reacts with tropylium fluoroborate 111 to provide azu-lene 113 [35]. The reaction is slow and it is necessary to use an acid scavenger so as to inhibit protiodesilylation by the fluoroboric acid that is generated during the course of the annulation. The excess tropylium salt abstracts a hydride from the reaction intermediate leading to the azulene. There are relatively few direct methods for the synthesis of azulenes. 

Tropones, Tropolones, and Tropylium Salts with Fused Heterocyclic Rings Part I Synthesis ... 

C. General Syntheses of Heterocyclic Fused Tropylium Salts. 129... 

D. Syntheses of Tropylium Salts Fused to Individual Heterocyclic Rings. 138... 

In some cases two isomeric tropones (4-oxo and 6-oxo) and two or more tropilidenes are formed from tropylium salts [66JCS(C)926] or from the equivalent pseudobases (cycloheptatrienols or tropols 69ZOB2601). Finally nucleophilic attack can generate an unstable tropol that yields, on oxidation, the desired tropone (78AJC1607). 

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